The invention relates to a method of checking for the presence of connection balls of components by illuminating the component with the connection balls and detecting reflections and shadows being produced.
In the course of automatically populating substrates, in particular printed circuit boards or ceramic substrates, with SMD components for surface mounting (SMD=Surface Mounted Devices), the components, before being emplaced, are visually inspected with regard to their position and the presence of all the connections, in particular all the connection balls in the case of ball grid arrays (BGAs), in order subsequently to correct the position of the component prior to emplacement on the substrate, or to sort out the component owing to missing connections.
WO 96/21343 discloses an apparatus for identifying the position of the connections of components which uses illumination with light which is obliquely incident on all sides, in order to automatically evaluate the reflections from connection balls and to determine the position of the component so that positional correction can subsequently be performed. In this case, use is made of segmented ring light illumination in order to illuminate all the connection balls uniformly from different sides.
However, this method does not make it possible to determine unambiguously whether all the connection balls are present at the intended contact points. Soldering residues from the fixing of the balls can also cause a reflection which resembles a reflection from a connection ball and simulates a ball that is present. The great dependence of the reflection position on the surface condition of the connection balls additionally makes the evaluation more difficult.
T. Ross et al: Inspection technique for solder reflow pad height/volume, IBM Technical Disclosure Bulletin, volume 22, No. 9, February 1980, page 4068, discloses a method of determining the height or the volume of connection balls on components, where an image-evaluating unit determines the shadow area cast by the connection balls and resulting from obliquely incident illumination. In this case, the illumination is effected only on one side, since if the illumination were on all sides, shadow areas cast by the connection balls and capable of evaluation would not be produced. Dark impurities on the components of the same size as shadow areas lead to misinterpretations concerning the presence of connection balls in the case of this method.